The present invention relates to a structure for sealing the gap between two concentric machine parts which are arranged to move relative to each other, the first machine part being provided with a groove while the second one comprises a plain contact surface arranged opposite the said groove, the sealing ring structure comprising a sealing ring made of a tough-elastic plastic material and fitted in the groove of the first machine part, with a circumferential surface of the said sealing ring facing the contact surface being provided with an annular recess so that the sealing ring bears upon the contact surface of the second machine part by sealing edges arranged near its ends, and comprising further a stressing ring of a rubber-elastic material arranged between the sealing ring and the bottom of the groove and loading the sealing ring in the radial direction, the axial cross sectional length of the said stressing ring being smaller than that of the sealing ring, and its axial position being fixed, relative to the sealing ring, in such a manner that it occupies a substantially central position between the two sealing edges.
A sealing ring structure of this type has been known from DE-PS 36 13 880. In the case of this sealing arrangement, the groove of the one machine part accommodating the sealing ring and the stressing ring is provided in its central area with a recess which is engaged by the stressing ring so that the axial position of the stressing ring is fixed by this recess while the axial position of the sealing ring is fixed by the groove. The production of a groove which is provided with a recess in its central area requires a considerably increased input of labor during manufacture of the sealing ring structure. In particular, such a design makes it impossible to use such a sealing ring structure in place of other sealing ring structures whose grooves intended to accommodate the sealing rings do not comprise any such recess. Although it would in principle be possible to arrange supporting bodies in the groove of conventional sealing ring structures, for supporting the stressing ring laterally, this would increase substantially the complexity of such a sealing ring structure.
The known seal belongs to the type comprising sealing edges arranged in series, which become active in operation so that the function is similar to that of a tandem seal. Consequently, certain quantities of the highly pressurized fluid which is to be sealed off, which may pass the sealing edge at the high-pressure side may gather in the space between the two sealing edges. In this manner, a considerable pressure rise may occur in the space between the two sealing edges, with the result that the sealing edges may be lifted off the contact surface of the second machine part, and a certain leakage may occur. Although the solution to interconnect the space between two sealing edges of the sealing ring with that side of the sealing ring which is in contact with the stressing ring, by a radial channel, has to be regarded as state of the prior art since the publication of patent application No. P 36 20 539.7, the stressing ring of the sealing ring structure described by patent application No. P 36 20 539.7 exhibits substantially the same axial length as the sealing ring, and the channel ends at a point near the high-pressure edge of the surface where the stressing ring is in contact with the sealing ring.